A firm winner

The behaviour of the landscape along the west coast of Sweden can sometimes be as dramatic as its looks, which has made life interesting for those working to stabilise the ground under new sections of the motorway linking Gothenburg with Oslo in Norway.

UK specialist contractor Keller's Swedish sister company LCM is dry soil mixing in soft ground that needs to be made stronger before a new section can be built for the E6 motorway.

As with much of Sweden's landscape, the ground prole comprises rocky granite outcrops with glacial quick clays, and the clays are very soft.

'Undisturbed, the quick clay's shear strength is between 20kPa and 40kPa. But if you disturb it that number goes down to zero, ' he says. 'It's not an easy terrain to build a road on.' In places where lime columns have been installed there is evidence of worked clay becoming so soft it has turned into liquid.

Designer Ramb÷ll decided to install thousands of lime columns to strengthen sections of a nearly 3km long stretch of ground along the line of what will be the new road. A lime cement powder binder mix, typically made up of 50% lime and 50% cement, is creating ground that is more than 3.5 times stronger than the required 100kPa working shear strength.

A spectacular landslide one evening last December at the south end of the site served as a stark reminder of what might happen to the new road without strengthening works.

Land shifted substantially, breaking up a portion of the E6 road, damaging Sweden's west coast railway line, lying to the east of the site, and causing the bed of a stream to collapse.

Nilsson says he made a visit to the site on the night of the landslide to see what damage had been done. But unable to see anything in the dark, he had to wait until the next day to see if the ground was still in place.

'If land where we had installed columns had moved, it would have caused problems because we would have been forced to make design changes, ' says Nilsson. 'Fortunately, it hadn't shifted.' Friction usually keeps quick clay hanging on to the smooth face of granite rock. But it is stable only until it is disturbed, most often by rainfall and water irrigating the soft ground.

Nilsson says typical landslides around this region occur because if groundwater increases, the smooth surface of the granite rock is lubricated.

This reduces friction between the rock and soil, which causes the soft clay to slide down the granite face. Although an investigation is under way to find the cause of December's event, Nilsson thinks this was the most likely reason, particularly as it followed a rainy period.

He says it was a stressful time trying to find out if the work completed by LCM had gone with the landslide. LCM had made amendments to the original design by consultant Ramb÷ll by installing 700mm diameter columns instead of 600mm at the south end of the site. This reduced the number of columns needed and was seen as safe because slopes at the south end are less steep than elsewhere about the site.

Seeing the stabilised area unaffected by the landslide reassured Nilsson and the team that lime columns would do the job of supporting the new road.

But the effect of the landslide on this relatively gradual incline was a timely warning about how much worse the effects could be in places to the north of the site where columns are being installed into much steeper slopes.

The area is divided into sections according to the direction in which a landslide might occur. Both 600mm and 700mm diameter columns are being installed in rows about 1m to 1.5m apart. Columns are installed in the ground at right angles against the direction of a potential landslide. Within rows, columns are installed at up to 1.1m centres and in places intersect one another with a 100mm overlap.

In each section, work began at the point furthest away from the slope and railway to the east of the site. Rig operators installed the columns, gradually working their way back towards the slope. Any displacement from the introduced binder would then force land back towards the slope, rather than away from it.

LCM advised main contractor Peab on the sequence to use to complete the dry soil mixing and a certain amount of land heave was expected.

An inclinometer helps the team monitor any tilting movement of the slope where work is being done, while a number of piezometers check for significant changes in groundwater pressure.

Because of the volume of lime cement being mixed with the soil, ground has been displaced by up to 500mm in places.

'Imagine if, in one row, you install 80 columns at 20m, then there is going to be a volume change in that area, ' says Nilsson.

He says this is because in places where columns intersect, they act as a buttress wall giving additional protection against future land movements. 'It's not just the dry soil mixing itself, but where we have used columns that makes the slope stable.' LCM's own drilling rigs with wide tracks are used to mix the lime and cement binder into the soft ground. According to Nilsson, the 30t rig used for much of the work so far is relatively lightweight to stop it sinking into quick clay.

Although rig weights were kept to a minimum, operators faced the additional challenge of transporting binder mix to the working area, while being careful not to cause unnecessary disturbance to unworked clay.

Timber sleepers are being used as a deck for the rigs to move along.

The mixing tool drills down to depth ? the columns range from 4m to 25m ? its three pairs of long narrow fl ights cutting through the soil to create air voids as it goes down. Each column is founded on rock.

Binder mix is then pneumatically pumped down the auger's hollow stem and out through a single hole just above the flights. The tool is retracted, turning at about 150rpm and blends 20kg/m and 30kg/m of lime cement powder in with the soil ? depending on the diameter of the column.

Water is not added to the binder because all the moisture needed to cure the columns is naturally extracted from moist clays around it.

Nilsson says within seven hours of a column being installed, the strength of the soil (now mixed with the binder) increases tenfold. However, site workers allow the column to cure for at least a week before the ground is built on.

Tests on the columns have proven a factor of safety of at least 3.5 times the 100kPa working load. But Nilsson says it is likely the columns may be stronger than this because LCM testing equipment can only measure up to 350kPa.

Keller sales and marketing director Martyn Singleton explains this strength is particularly high and although a more economical method using less binder mix to create weaker columns could have been employed, it was better to go for the high strength option because the soils are so sensitive and the risk of landslides so high.

LCM started work on the SEK20.87M ($3M) subcontract to install 417,250m of lime columns in May last year and expects to be finished by this month. Peab is building roads, two bridges and an underpass in this section and Nilsson says he expects this part of the E6 to be open by 2008. The works, for client Võgverket, the Swedish Road Administration, are part of extensive improvements to the route to cope with an increase in traffic.

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